1. Field of Invention
This invention relates to a method and an apparatus for processing, measuring or storing high-speed optical signals.
2. Description of the Prior Art
An example of prior art optical signal processing circuit is shown in FIG. 1. In the Figure, the reference numerals 201 and 205 indicate diffraction gratings, 202 and 204 are lenses, and 203 is a spatial filter or an optical storage medium. When a time series signal light is applied to his optical circuit, a Fourier transformation of the time series signal light, that is, a frequency spectrum distribution thereof is formed on the spatial filter 203 by the frequency decomposition function of the diffraction grating 201 and the Fourier transformation function of the lens 202. When the frequency spectrum distribution is modulated by means of the spatial filter 203, the waveform of the time series signal can be modulated. Here the wave form can be controlled by the spatial filter 203 even when the time series signal is very high in speed.
As an example, when an optical signal of a pulse width of 200 fs and a pulse interval of 5 ps shown in the upper part of FIG. 2 is applied, the incident optical spectrum has a shape as shown in the upper part of FIG. 3, having an optical power distribution shown by the broken line in the middle part of FIG. 3 on the spatial filter 203 after passing through the diffraction grating 201 and the lens 202, and when this is modulated by the spatial filter 203, it has a shape as shown in the lower part of FIG. 3 in the spectrum after passing through the spatial filter 203. A time-dependent waveform corresponding to the spectrum is the pulse sequence shown in the lower part of FIG. 3. Thus, optical signal processing can be achieved by modulating the frequency spectrum of optical signal by the spatial filter 203. That is, various waveform shaping according to the filter is possible.
Further, with 203 shown in FIG. 1 used as an optical storage medium, by applying time series signal light and reference light simultaneously, an interference fringe of both lights is hologram recorded on the optical storage medium 203. After the recording, when only the reference light is incident, the signal light is reproduced and output. Such studies are reported, for example, in A. M. Weiner, xe2x80x9cProgramable shaping of Femtosecond optical pulses by use of 128-Element Liquid Crystal Phase Modulator,xe2x80x9d IEEE J Quntun Electronics, Vol . 28 No. 4, pp. 908-920 (1992); A. Weiner et al., xe2x80x9cSpectral holography of Shaped femtosecond pulsesxe2x80x9d, Optics Letters, vol. 17, pp. 224-226 (1992).
With the advance in optical communication technology, pulse widths of optical signals utilized in optical transmission are 100 ps (ex; FA-10G system) in the practical application stage, and those of next-generation very large capacity transmission apparatus are considered to utilize picosecond pulses of 1-10 ps. Optical pulses of femtosecond region are application region of the time being in the research and development and material evaluation of stable light sources, and considered not to be applied in optical communications immediately. That is, basic apparatus and method enabling optical pulse generation, waveform shaping, waveform measurement, waveform recording, correlation processing, and the like are required for constructing next-generation very high capacity systems.
However, the above-described prior art has the following problems. That is, in the modulation or hologram recording, all of the diffraction gratings 201, 203, the lenses 202 and 204, and the spatial filter 203 must be laid out in high precision, are liable to be affected by the external environment, are thus difficult to be modular structured, and are nearly impossible to operate other than in a so-called experimental-environment. Therefore, it is not practicable at present stage.
Still further, when treating a time series signal, signal processing is in principle possible by single dimensional diffraction grating and lens. However, the diffraction grating and the lens have a redundant two-dimensional structure, which requires tedious positioning which is inherently unnecessary.
Yet further, when treating a long pulse sequence of over 10 ps or a pulse of large pulse width, it is required to increase the incident beam diameter which, in turn, requires large-sized diffraction grating or lens, and thus a large sized apparatus.
That is, the prior art structure using the diffraction grating pair and the lens effective for femtosecond pulses requires a very large sized apparatus for picosecond pulses, and is difficult to be packaged in a transmission apparatus of about 30xc3x9740xc3x973 cm. Further, it is required to use a connection optical system with the optical fiber, and flexible apparatus design according to the pulses is Impossible.
Heretofore, a semiconductor mode-locked laser is known as picosecond pulse generation means.
FIG. 4 shows the structure of a prior art mode-locked laser for use as a short pulse light source.
In the Figure, the mode-locked laser comprises an optical gain medium 51, an pumping circuit 52 for forming a population inversion to the optical gain medium 51, mirrors 53-1 and 53-2 constituting an optical resonator, an optical modulator 54 placed in the optical resonator, and a clock generator 55 for driving the optical modulator 54. In this construction, when the clock generator 55 drives the optical modulator 54 at a clock frequency equal to the resonance mode spacing of the optical resonator or an integer multiple thereof, an optical short pulse sequence of a repetition frequency equal to the clock frequency or an integer multiple thereof.
FIG. 5 shows the structure of a multi-wavelength light source for simultaneously oscillating light of a plurality of wavelengths.
In the Figure, the multi-wavelength light source comprises an optical gain medium 61, an arrayed-waveguide grating 62, a lens 63 for coupling the optical gain medium 61 with the arrayed-waveguide grating 62, a high reflection mirror 64 and a low reflection mirror 65 disposed at both end surfaces of the optical gain medium 61, and a high reflection mirror 66 disposed at the other end of the arrayed-waveguide grating 62.
The arrayed-waveguide grating 62 comprises an input waveguide 71, an arrayed waveguide 73 including a plurality of waveguides gradually increasing in length by a waveguide length difference xcex94L, a plurality of output waveguides 75, a slab waveguide 72 for connecting the input waveguide 71 and the arrayed waveguide 73, and a slab waveguide 74 for connecting the arrayed waveguide 73 and the output waveguide 75, which are formed on a substrate 70.
Light incident into the input waveguide 71 spreads by diffraction in the slab waveguide 72, and incident and distributed in equal phase into individual waveguides of the arrayed waveguide 73. The light-transmitted in the individual waveguides of the arrayed waveguide 73 and reaching the slab waveguide 74 has a phase difference corresponding to the waveguide length difference xcex94L. Since the phase difference varies with the wavelength, when focused on the focal plane by the lens effect of the slab waveguide 74, the light is focused at different positions by wavelengths. Therefore, light of different wavelengths are taken out from the individual waveguides of the output waveguide 75.
In the multi-wavelength light source using such an arrayed waveguide grating 62, an optical resonator is formed between the high refection mirror 64 and the high reflection mirror 66, and light of a plurality of wavelengths can be simultaneously oscillated by steadily exciting the optical gain medium 61.
However, the prior art mode-locked laser has the following problems.
(1) The oscillation mode envelope spectrum is largely varied by the operation condition, and it is difficult to set the central wavelength and the pulse width.
(2) Since the amplitude and phase of each mode cannot be independently controlled, pulse shape design is difficult.
(3) A very large number of modes are excited, however, correlation between modes is insufficient due to dispersion and nonlinear effect of the semiconductor medium of the long resonator, and it is difficult to generate a transform limit optical short pulse sequence.
Further, since the phase to each mode is not controlled in the multi-wavelength as shown in FIG. 5, it is impossible to generate an optical short pulse sequence of high repetition frequency by mode locking.
As described above, there has heretofore been a semiconductor mode-locked laser as picosecond pulse generation-means, however, to utilize it as a light source for optical communications, it is required that the phase and intensity are stable, the central wavelength and pulse width and pulse shape can be set (in design and fabrication), and a high quality pulse close to the transform limit is generated. However, present semiconductor mode-locked lasers are difficult to simultaneously meet these requirements. Further, it is very difficult to incorporate the prior art shown in FIG. 1 in a semiconductor mode-locked light source, and no studies have been reported on the problems.
In a very high speed optical transmission apparatus, distortion of waveform due to group velocity dispersion in the optical fiber is a first factor that limits the transmission distance. Dispersion characteristics of a transmission line (optical fiber) depend on ambient temperature, on material and cover with passage of time. Further, the dispersion characteristics are changed when the optical fiber is changed to another optical fiber in association with a malfunction or replacement of the transmission line. Or. even if the dispersion of the optical fiber is unchanged, the dispersion value applied to the optical signal is changed by changes in light source wavelength or filter characteristics.
Since even dispersion shift optical fibers of small dispersion, which are generally used, have a dispersion of about xc2x11 ps/nm/km, a transmission section of 80 km has a dispersion of xc2x180 ps/nm. Since an optical band width of optical signal of pulse width of 10 ps at 20 Gbit/s is about 1 nm, a pulse broadening of a maximum of 80 ps is generated. However, the time slot of 20 Gbit/s signal is 50 ps, a large inter-symbol interference is generated to produce large errors. Therefore, an apparatus for compensating (equalizing) the dispersion of transmission line is indispensable for a very high speed transmission apparatus.
An example of prior art is shown in FIG. 6. In FIG. 6, 01 is an optical amplifier, 02 is an optical switch, and 03 is a dispersion compensation fiber.
In the prior art, the optical signal is passed through another optical fiber having dispersion characteristics reverse to the dispersion of the transmission line to compensate the dispersion, thereby obtaining a good waveform.
Since dispersion characteristics of the dispersion compensation fiber 03 are not variable, it is general to provide fibers of different dispersion characteristics for compensating the dispersion according to changes in dispersion characteristics of the transmission line.
However, the prior art has the following problems.
(i) compensation is difficult for high order dispersion.
(ii) A number of fibers must be provided to compensate the dispersion. In particular, since the tolerance of dispersion is narrow in a very high speed optical signal, fibers of small increment in dispersion value are required. As a result, the apparatus becomes large in size, and an optical switch of multiple switches is required.
(iii) Since the dispersion compensation fiber is switched by an optical switch, a momentary disconnection of optical signal occurs during switching.
Further, as other prior art examples, there are constructions of chirped fiber grating and multiple-connected MZ interferometer, however, these have the. following problems.
(iv) Control width of central wavelength of dispersion compensation is small, and the compensation band width is narrow.
In a very high speed optical transmission apparatus, a second factor of limiting the transmission distance is distortion of waveform due to self-phase modulation in the optical fiber. Another example of prior art is shown in FIG. 7. In FIG. 7, the symbol 04 indicates an optical transmitter, 05 is a high dispersion fiber, 06 is an optical-amplifier, 07 is an optical transmission line, 08 is a dispersion compensation circuit, and 09 is an optical receiver.
This construction provides a transmission method in which an optical signal is previously passed through a high dispersion medium on the assumption of compensation to increase the pulse width, thereby reducing the self-phase modulation. Since the self-phase modulation is generated nearly proportional to the optical pulse peak intensity, the modulation can be reduced by increasing the pulse width to decrease the peak power. Since, dispersion can be compensated to reproduce the waveform, however, degradation of waveform due to self-phase modulation, which is a nonlinear phenomenon, cannot be reproduced by a normal linear waveform equalization method, a transmission apparatus is required which does not generate a nonlinear phenomenon in the transmission line.
However, the prior art has the following problems.
(v) Since changes in dispersion in the transmission line cannot be previously estimated, there is a possibility that dispersion is equalized by chance in the course of the transmission line, resulting in self-phase modulation leading to an increase in waveform degradation and errors.
As described above, as picosecond pulse waveform shaping (e.g., dispersion compensation) means, there are other optical fibers having reverse dispersion characteristics to the dispersion in the transmission line, a chirped fiber grating, and a multiple-connected MZ interferometer. However, these means are difficult to achieve compensation for high order dispersion, variable dispersion compensation, and wide band compensation. Further, the prior art shown in FIG. 1 is very small in compensatable dispersion amount. For example, for an optical pulse of 2 ps in pulse width, it can provide only a compensation of about 5 ps/nm.
The optical signal processing apparatus as described above is required to measure waveform of optical signals of higher speed.
Heretofore, as waveform measuring and recording means, there is a very high speed O/E converter or a streak camera. However, the band width of the O/E converter is as much as 50 GHz and impossible to measure picosecond pulses of 1-10 ps. Further, since the streak camera is low in sensitivity in the optical communication wavelength region, a sufficient S/N is not obtained by a single sweeping, and a real-time waveform cannot be observed. There is no report of study using the prior art shown in FIG. 1. When the prior art is applied, as is, since light is distributed two-dimensionally on the Fourier transformation plane, measurement of high S/N is difficult unless a specific optical system is devised.
An object of the present invention, as described above, is to provide an optical signal processing apparatus and optical signal processing method which enables generation, waveform shaping, waveform measurement, waveform recording, correlation processing, and the like of optical pulses of 1-10 ps.
Basic construction of the optical signal processing apparatus according to the present invention suitable for such problems comprises an optical waveguide, first means for equally distributing an output of the optical waveguide, a arrayed waveguide comprising an aggregate of optical waveguides changing in optical length by a constant interval for spectrally dividing the output light, second means for focusing the optical output of the arrayed waveguide, and a mirror for receiving and reflecting incident light focused by the second means.
Or, the optical signal processing apparatus according to the present invention comprises an optical waveguide, first means for equally distributing an output of the optical waveguide, a arrayed waveguide for dividing the output light, second means for focusing the optical output of the arrayed waveguide, and a spatial filter for receiving incident light focused by the second means and spectrally dividing the incident light on a straight line and for modulating the incident light into a desired amplitude or phase according to the position on the straight line, thereby reflecting the incident light.
Further, another construction of the optical signal processing apparatus according to the present invention comprises a first optical waveguide, first means for equally distributing an output of the first optical waveguide, a first arrayed waveguide comprising an aggregate of optical waveguides changing in optical length by a constant interval for dividing the output light, second means for focusing the optical output of the first arrayed waveguide, a spatial filter for receiving incident light focused by the second means and distributing the incident light on a straight line and for modulating the light into a desired amplitude according to the position on the straight line, third means comprising an aggregate of optical waveguides changing in optical length by a constant interval for applying light to the second arrayed waveguide, fourth means for converging the output light of the second arrayed waveguide to a single point, and a second optical waveguide to which the output light of the fourth means is applied.
A yet further construction of the optical signal processing apparatus according to the present invention comprises a reflective type spatial filter, a arrayed waveguide comprising an aggregate of optical waveguides changing in optical length by a constant interval, first means for applying coherent light to the reflective type spatial filter and inputting the coherent light modulated by the reflective type spatial filter into the arrayed waveguide, and second means for converging output light of the arrayed waveguide to a single point.
A yet further construction of the optical signal processing apparatus according to the present invention comprises a transmission type spatial filter, a arrayed waveguide comprising an aggregate of optical waveguides changing in optical length by a constant interval, first means for applying coherent light to the transmission type spatial filter, second means for applying the coherent light modulated by the transmission type spatial filter into the arrayed waveguide, and third means for converging output light of the arrayed waveguide to a single point.
The optical signal processing method according to the present invention is characterized in that an optical signal is input to an optical signal processing apparatus having a arrayed waveguide and a spatial filter, to convert the optical signal into a frequency spectral image, the frequency spectral image is subjected to a desired modulation by the spatial filter, and the modulated frequency spectral image is converged to a single point to obtain a new optical signal.
In the optical signal processing method, the optical signal processing apparatus used is preferably the above-described optical signal processing apparatus.
Another arrangement of the optical signal processing method according to the present invention is characterized in that coherent light is input in an optical signal processing apparatus having a arrayed waveguide and a spatial filter written with a hologram image corresponding to the frequency spectrum of a desired optical signal to generate an optical signal.
Still further, in the present invention, by combining the optical signal processing apparatus of the basic structure with predetermined parts, generation of a short pulse is possible.
The basic construction of such optical signal processing apparatus is characterized by comprising in the optical signal processing apparatus of the basic structure having the above-described mirror, at the input side of the optical waveguide, optical modulation means for modulating an oscillation light in an optical resonator with a frequency nearly equal to the resonance mode spacing or an integer multiple thereof, and optical gain means, wherein the modulation means, the gain means, and the optical waveguide are sequentially coupled by optical coupling means, an optical reflection mirror is disposed on the end surface not facing the coupling means of the optical modulator, the mirror formed on the end surface of the second means is a high reflection type, a resonator is formed between these high reflection type mirrors, thereby enabling generation of a short pulse light.
In the construction for generating short pulses, a plurality of output waveguides may be disposed at predetermined spacings between the focal plane of one slab waveguide of the arrayed waveguide grating and the high reflection mirror.
Similarly, the plurality of output waveguides may be disposed at equal spacings.
Similarly, the high reflection mirrors corresponding to the individual output waveguides may be different in reflectivity from each other.
Similarly, the plurality of output waveguides may be set with predetermined waveguide length differences for compensating dispersion in the optical resonator.
Similarly, in place of the high reflection mirrors, a plurality of lens arrays disposed at a predetermined spacing, and a liquid crystal light modulator having a high reflection mirror on one side may be provided.
Similarly, an optical synthesizer for synthesizing part or all of a plurality of output waveguides.
Similarly, gratings formed in the individual output waveguides may be different in diffractive efficiency from each other.
Similarly, a plurality of high reflection mirrors may be disposed at predetermined spacings on the focal plane of one slab waveguide of the arrayed waveguide grating.
Similarly, a plurality of high reflection mirrors may be disposed at equal spacings.
Similarly, the plurality of high reflection mirrors may be different in reflectivity from each other.
Similarly, in place of the high reflection mirrors, a plurality of diffraction gratings may be formed at a predetermined spacing in the focal plane of the slab waveguide.
Similarly, the plurality of diffraction gratings may be disposed at equal spacings.
Similarly, the plurality of diffraction gratings may be different in diffractive efficiency from each other.
Similarly, the individual positions of the plurality of diffraction gratings may be dislocated in the normal direction of the focal plane of the slab waveguide.
Similarly, in place of the high reflection mirrors, a groove may be formed on the focal plane of the slab waveguide, and films formed by stacking a plurality of mirrors may be disposed at a predetermined spacing in the groove.
Similarly, the optical modulation means and the optical gain means may be integrated.
Similarly, part of the individual components may be connected with an optical fiber.
Similarly, the position of the high reflection mirror may be controlled by a fine control mechanism.
Similarly, the spacing of the high reflection mirrors may be varied in the normal direction of the slab waveguide.
Further, based on the optical signal processing apparatus of the basic structure according to the present invention, an apparatus for observing the waveform of the optical signal to be processed can be constructed.
Such a waveform observable optical signal processing apparatus comprises an optical waveguide, an arrayed waveguide comprising a plurality of optical waveguides gradually increasing in waveguide length, distribution means for distributing output light of the optical waveguide to the arrayed waveguide, focusing means for focusing the output light of the arrayed waveguide, a spatial filter disposed in the vicinity of the focal plane of the focusing means for modulating the light image, reflection means for reflecting light modulated by the spatial filter, and optical division means for taking out the reflected light from the reflection means in the optical waveguide.
Or, the apparatus comprises a first optical waveguide, a first arrayed waveguide comprising a plurality of optical waveguides gradually increasing in waveguide length, distribution means for distributing the output light of the first optical waveguide to the first arrayed waveguide, first focusing means for focusing the output light of the first arrayed waveguide, a spatial filter disposed in the vicinity of the focal plane of the first focusing means for modulating the light image, and further comprising a second arrayed waveguide comprising a plurality of optical waveguides gradually increasing in waveguide length, second focusing means for focusing the light modulated by the spatial filter to the second arrayed waveguide, a second optical waveguide, and wave synthesis means for synthesizing the output light of the second arrayed waveguide and coupling to the second optical waveguide.
Still further, the apparatus comprises a first optical waveguide, a first arrayed waveguide comprising a plurality of optical waveguides gradually increasing in waveguide length, distribution means for distributing output light of the first optical waveguide to the first arrayed waveguide, a reference light input optical waveguide, a second arrayed waveguide comprising a plurality of optical waveguides gradually increasing in waveguide length, second distribution means for distributing the output light of the reference light input optical waveguide to the second arrayed waveguide, focusing means for focusing the output light of the first arrayed waveguide and the output light of the second arrayed waveguide, an optical recording medium disposed in the vicinity of the focal plane of the focusing means, and optical division means for taking out the reflected light from the first optical waveguide.
A yet further construction of the waveform observable apparatus comprises a first optical waveguide, a first arrayed waveguide comprising a plurality of optical waveguides gradually increasing in waveguide length, first distribution means for distributing the output light of the first optical waveguide to the first arrayed waveguide, a third arrayed waveguide comprising a plurality of optical waveguides gradually increasing in waveguide length, second distribution means for distributing the output light of the reference light input optical waveguide to the third arrayed waveguide, first focusing means for focusing the output light of the first arrayed waveguide and the output light of the third arrayed waveguide, an optical recording medium disposed in the vicinity of the focal plane of the first focusing means, a second arrayed waveguide comprising a plurality of optical waveguides gradually increasing in waveguide length, second focusing means for focusing the light modulated by the optical recording means to the second arrayed waveguide, a second optical waveguide, and wave synthesis mans for synthesizing the output light of the second arrayed waveguide and coupling to the second optical waveguide.
Yet further, another construction of the waveform observable apparatus comprises a first optical waveguide, a first arrayed waveguide comprising a plurality of optical waveguides gradually increasing in waveguide length, first distribution means for distributing the output light of the first optical waveguide to the first arrayed waveguide, a first reference light input optical waveguide, a second arrayed waveguide comprising a plurality of optical waveguides gradually increasing in waveguide length, second distribution means for distributing the output light of the first reference light input optical waveguide to the second arrayed waveguide, a second reference light input optical waveguide, first focusing means for focusing the output light of the first arrayed waveguide and the output light of the second arrayed waveguide and the output light of the second reference light input optical waveguide, an optical recording medium disposed in the vicinity of the focal plane of the first focusing means, second focusing means for focusing the light modulated by the optical recording medium, and a light receiver array disposed in the vicinity of the focal plane of the second focusing means.
In the waveform observable apparatus, the focusing means may be a slab waveguide having a circular end surface.
Similarly, the focusing means may comprise a slab waveguide and a phase spatial modulation device.
Similarly, the spatial filter may be a phase filter, or an amplitude filter, or a spatial filter formed of amplitude filters and phase filters connected in multiple stages.
Similarly, the focal length of the phase spatial modulation device may be equal to the focal length of the slab waveguide of the coupling means.
Similarly, the spatial filter and the reflection means may be combined to form a pattern mirror comprising a number of partial mirrors.
Similarly, the spatial filter may be a spatial filter combining with a function of a phase spatial modulation device.
Similarly, the focusing means may be a lens.
Similarly, the focusing means may be a slab waveguide of the focal plane, having a phase adjusting arrayed waveguide at the end of the slab waveguide, and the phase adjusting arrayed waveguide end may be connected to the spatial filter.
Similarly, the focusing means may be a slab waveguide of the focal plane, having a phase adjusting arrayed waveguide at the slab waveguide end, and the phase adjusting arrayed waveguide may have an optical modulator array.
Similarly, phase difference between waveguides of the phase adjusting arrayed waveguide may be an integer multiple of 2xcfx80.
Similarly, the optical division means may be an optical circulator.
Similarly, the focusing means is a slab waveguide, having optical bend means at the slab waveguide end for bending light in direction perpendicular to the waveguide.
Similarly, the spatial filter may be a liquid crystal spatial modulator comprising a glass substrate, a transparent electrode, a liquid crystal, and a liquid crystal alignment film.
Similarly, a quarter-wave plate may be provided in the liquid crystal spatial modulator.
Similarly, the liquid crystal of the liquid spatial modulator may be a twist nematic type.
Further, the optical signal processing method by the waveform observable optical signal processing apparatus is characterized in that a time series optical signal is input in the optical waveguide to convert the time series optical signal into a frequency spectrum image, the frequency spectrum image is subjected to desired phase or amplitude or both modulations, and the modulated light is synthesized to obtain a new time series optical signal. Or, filter characteristics of the spatial filter may be a hologram image of a pattern corresponding to the frequency spectrum of a desired time series optical signal, and coherent pulse light is applied to the optical waveguide to generate the desired optical signal. Still further, an optical signal may be input in the optical waveguide, a reference light of coherent pulse light is applied to the reference light input waveguide, hologram recording is made on the recording medium, a reference light of another coherent pulse light is applied to the reference light input waveguide, to output a phase conjugate light of the signal light. Yet further, signal light is input in the optical waveguide, a reference light of coherent pulse light is applied to the reference light input waveguide, hologram recording is made on the recording medium, no a reference light of another coherent pulse light is applied to the reference light input waveguide, to output a signal light or a correlated light of the signal light and the reference light. Yet further, a signal light is input in the optical waveguide, a reference light of coherent pulse light is applied to the first reference light input waveguide, a reference light of single color is applied to the second reference light input waveguide to form reference light of coherent pulse light in the first reference light input waveguide to observe the pulse waveform.
Yet further, the present invention, based on the optical signal processing apparatus of the above-described basic structure, can provide an optical signal processing apparatus which enables dispersion compensation of the processed optical signal. The optical signal processing apparatus that enables such dispersion compensation comprises a first optical amplifier, an optical wavelength filter, a first optical waveguide, a first arrayed waveguide comprising a plurality of optical waveguides gradually increasing in waveguide length, distribution means for distributing the output light of the first optical waveguide to the arrayed waveguide, first focusing means for focusing the output light of the first arrayed waveguide, a spatial filter disposed in the vicinity of the focal plane of the first focusing means for modulating optical image, reflection means for reflecting light modulated by the spatial filter, optical division means for taking out the reflected light from the first optical waveguide, and a second optical amplifier.
Another construction of the dispersion compensatable optical signal processing apparatus comprises a first optical amplifier, an optical wavelength filter, a first arrayed waveguide comprising a plurality of optical waveguides gradually increasing in waveguide length, distribution means for distributing the output light of the first optical waveguide, first focusing means for focusing the output light of the first arrayed waveguide, a spatial filter disposed in the vicinity of the focal plane of the first focusing means for modulating the optical image, a second arrayed waveguide comprising a plurality of optical waveguides gradually increasing in waveguide length, second focusing means for focusing light modulated by the spatial filter to the second arrayed waveguide, a second optical waveguide, optical synthesis means for synthesizing the output light of the second arrayed waveguide and coupling to the second optical waveguide, and a second optical amplifier.
Yet further, an optical signal processing apparatus can be constituted of the dispersion compensatable optical signal processing apparatus, a light source, and an optical modulation signal generation circuit.
Yet further, an optical signal processing apparatus can be composed of the dispersion compensatable optical signal processing apparatus and an optical receiver.
Similarly, an optical signal processing apparatus can comprise the dispersion compensatable optical signal processing apparatus, an optical signal transmitter circuit comprising a light source, an optical modulator, an optical signal receiver circuit comprising the dispersion compensatable optical signal processing apparatus and an optical receiver, and an optical transmission line.
In the dispersion compensatable optical signal processing apparatus, the spatial filter may be a phase filter in which a relative phase xcfx86 may have a characteristic approximating
xcfx86(x)=Mod[ax2, xcfx80]xe2x80x83xe2x80x83(a: constant)
with respect to the position (x) on the spatial filter. (wherein Mod[u, v] indicates a remainder using v as a modulus.).
Similarly, the spatial filter may be a phase filter in which a relative phase xcfx86 may have a characteristic approximating
xcfx86(x)=Mod[ax2, 2xcfx80]xe2x80x83xe2x80x83(a: constant)
with respect to the position (x) on the spatial filter. (wherein Mod[u, v] indicates a remainder using v as a modulus.).
Similarly, the spatial filter may be a phase filter in which a relative phase xcfx86 has a characteristic approximating
xcfx86(x)=xcfx80/2 (x greater than 0) and xcfx86 (x)=0 (x less than 0), or
xcfx86(x)=0 (x greater than 0) and xcfx86(x)=xcfx80/2 (x less than 0)
with respect to the position (x) on the spatial filter, thereby achieving amplitude modulationxe2x80x94angular modulation conversion.
Similarly, the spatial filter may be a phase filter in which a relative phase xcfx86 has a characteristic approximating
xcfx86(x)=xcfx80(x greater than 0) and xcfx86(x)=0 (x less than 0), or
xcfx86(x)=0 (x greater than 0) and xcfx86(x)=xcfx80 (x less than 0)
with respect to the position (x) on the spatial filter, thereby achieving amplitude modulationxe2x80x94angular modulation conversion.
Yet further, in the dispersion compensatable optical signal processing apparatus, the spatial filter may comprise a phase filter and an amplitude filter.
Similarly, the spatial filter may be a liquid crystal spatial modulator comprising a glass substrate, a transparent electrode, a liquid crystal, and a liquid crystal alignment film.
Yet further, an optical signal processing method by the above dispersion compensatable optical signal processing apparatus is characterized in that frequency spectral chase of optical signal generated by the optical signal transmitter circuit is modulated, and dispersion in the optical fiber and frequency spectral phase modulation by the optical signal transmitter circuit are compensated by the optical receiver circuit.
Another arrangement of the optical signal processing method is characterized in that the frequency spectral amplitude of the optical signal generated by the optical signal transmitter circuit is modulated, dispersion in the optical fiber and frequency spectral amplitude modulation by the optical signal transmitter circuit are compensated by the optical signal receiver circuit.
Similarly, a further arrangement is characterized in that frequency spectral phase and frequency spectral amplitude of the optical signal generated by the optical signal transmitter circuit are modulated, and dispersion in the optical fiber and frequency spectral phase modulation and frequency spectral amplitude modulation are compensated by the optical receiver circuit.
A yet further construction of the dispersion compensatable optical signal processing apparatus comprises a first optical signal processing apparatus comprising:
a short pulse light source;
a first optical amplifier;
a first optical wavelength filter;
a first optical splitting means for dividing the output light of the first optical wavelength filter into n(integer) units of light;
first n units of optical modulation circuits;
a first arrayed waveguide comprising a plurality of optical waveguides gradually increasing in waveguide length;
first distribution means for distributing the output light of the first n units of input/output optical waveguides to the first arrayed waveguide;
first focusing means for focusing the output light of the first arrayed waveguide;
a first spatial filter disposed in the vicinity of the focal plane of the first focusing means for modulating light image;
first reflection means for reflecting light modulated by the first spatial filter;
optical splitting means for taking out the reflected light from the first n units of input/output optical waveguides;
optical combining means for synthesizing the reflected light from the n units of second optical splitting means; and
a second optical amplifier;
and a second optical signal processing apparatus comprising:
an optical transmission line;
a third optical amplifier;
a second optical wavelength filter;
third optical splitting means for dividing the output light of the second optical wavelength filter into n (integer) units of light;
second n units of input/output optical waveguides;
a second arrayed waveguide comprising a plurality of optical waveguides gradually increasing in waveguide length;
second distribution means for distributing the output light of the second input/output optical waveguides to the second arrayed waveguide;
second focusing means for focusing the output light of the second arrayed waveguide;
a second spatial filter disposed in the vicinity of the focal plane of the second focusing means for modulating light image;
second reflection means for reflecting light modulated by the second spatial filter;
fourth optical splitting means for taking out the reflected light from the second input/output optical waveguide; and
n units of optical receivers for receiving the reflected light from the n units of fourth optical splitting means.
A yet further construction of the dispersion compensatable optical signal processing apparatus comprises a first optical signal processing apparatus comprising:
a short pulse light source;
a first optical amplifier;
a first optical wavelength filter;
a first optical splitting means for dividing the output light of the first optical wavelength filter into n(integer) units of light;
first n units of optical modulation circuits;
first n units of input/output optical waveguides a first arrayed waveguide comprising a plurality of optical waveguides gradually increasing in waveguide length;
first distribution means for distributing the output light of the first input/output optical waveguides to the first arrayed waveguide;
first focusing means for focusing the output light of the first arrayed waveguide;
a first spatial filter disposed in the vicinity of the focal plane of the first focusing means for modulating light image;
a second arrayed waveguide comprising a plurality of optical waveguides gradually increasing in waveguide length;
second focusing means for focusing the light modulated by the first spatial filter to the second arrayed waveguide;
first n units of output optical waveguides;
first optical combining means for synthesizing the output light of the second arrayed waveguide and coupling to the first n units of output optical waveguides;
second optical combining means for coupling output of the first n units of output optical waveguides;
a second optical amplifier; and a second optical signal processing apparatus comprising:
an optical transmission line;
a third optical amplifier;
a second optical wavelength filter;
third optical splitting means for dividing the output light of the second optical wavelength filter into n (integer) units of light;
second n units of input/output optical waveguides;
a third arrayed waveguide comprising a plurality of optical waveguides gradually increasing in waveguide length;
second distribution means for distributing the optical waveguide output light of the second input/output optical waveguide to the third arrayed waveguide;
third focusing means for focusing the output light of the third arrayed waveguide;
a second spatial filter disposed in the vicinity of the focal plane of the third focusing means for modulating light image;
a fourth arrayed waveguide comprising a plurality of optical waveguides gradually increasing in waveguide length;
fourth focusing means for focusing light modulated by the second spatial filter to the fourth arrayed waveguide;
second n units of output optical waveguides;
third optical coupling means for coupling the output light of the fourth arrayed waveguide and coupling to the second n units of output optical waveguides; and
n units of optical receivers for receiving output light from the second output optical waveguide.
Yet further, the present invention, based on the optical signal processing apparatus of the above-described basic structure, can provide an optical signal processing apparatus which enables real-time observation of waveform of the processed optical signal.
Such a waveform observable optical signal processing apparatus comprises first timexe2x80x94space conversion means for converting time series optical signal of the signal light into spatial signal light, second timexe2x80x94space conversion means for converting time series optical signal of reference light into spatial signal light, focusing means for focusing the spatial signal light individually output from the first timexe2x80x94space conversion means and the second timexe2x80x94space conversion means to make interference with each other, light receiving means disposed in the vicinity of the focal plane of the focusing means for receiving an interference light image of a plurality of optical signals incident into the focusing means, and an optical signal restoration circuit for restoring a time series signal of the signal light from the detection signals of the light receiving means.
In the waveform observable optical signal processing apparatus, the first and second timexe2x80x94space conversion means may comprise arrayed waveguide gratings, and the focusing means may comprise a slab waveguide having a function for making Fourier transformation of the spatial signal light.
Similarly, in the waveform observable optical signal processing apparatus, the first and second timexe2x80x94space conversion means may comprise diffraction gratings, and the focusing means may comprise a lens for making Fourier transformation of the spatial signal light.
Similarly, in the waveform observable optical signal processing apparatus, the light receiving means may comprise a photodiode array.
Similarly, in the waveform observable optical signal processing apparatus, the optical signal restoration circuit may restore electric field distribution of optical pulse of the signal light by calculation from electric field distribution of the Fourier transform hologram detected by the light receiving means.
A signal processing method by the waveform observable optical signal processing apparatus is characterized by comprising a step of converting a time series optical signal of unknown signal light into spatial signal light, a step of converting time series optical signal of known reference light into spatial signal light, a step of focusing individually the spatial signal of the signal light and the spatial signal of the reference light to make interference with each other, thereby forming a hologram image of a pattern corresponding to the frequency spectrum of time series signal, a step of receiving the hologram image to be converted into an electrical signal, and a step of restoring the unknown signal light from the hologram image converted to electrical signal using a predetermined calculation formula.
In the optical signal processing method, the step of restoring the unknown signal light from the hologram image may have a mathematical calculation operation for multiplying the hologram image formed on the focal plane with electric field distribution of reconstructing light mathematically derived from the electric field distribution of known reference light, followed by Fourier transformation operation and spacexe2x80x94time conversion operation.
Further, in the signal processing method, the electric field distribution of reconstructing light on the focal plane to be multiplied with the hologram image may include a factor of dividing by the square of absolute value of amplitude distribution of electric field distribution at the focal plane of the known reference light.